From what you described, it does seem a bit curious that he insists on attending these events alone every single time especially considering that the two of you aren't a brand new couple. To me, it seems like a red flag that should not be ignored.
Of course it is perfectly normal for people to want to do certain things without their significant others once in a while but regularly going solo to social gatherings is something that people generally do when they are single and hoping to mingle.
In my opinion, the best thing you can do is to simply tell him how this behavior of his makes you feel. Be careful in the way that you approach him though. Don't make any accusations or point fingers but instead, just let him know that you are feeling somewhat left out. Explain that while you are okay with the idea of him hanging out with only his friends every now and then, you also need him to make some sort of effort to include you in those plans once in a while.
What you do from there depends largely on his reaction to the conversation. Healthy relationships are built on a foundation of communication, trust, and compromise. If he truly cares about you and wants to make the relationship work, then he should take your feelings into consideration and be willing to compromise with you on this issue. Otherwise, you might be better off cutting your losses and searching for a more suitable partner; one who understands and appreciates your feelings and needs, as well as his own.
Whatever happens, I wish you the best of luck!
